Hydrogen passivation shut down system for a fuel cell power plant

ABSTRACT

The invention is a hydrogen passivation shut down system for a fuel cell power plant ( 10 ). An anode flow path ( 24 ) is in fluid communication with an anode catalyst ( 14 ) for directing hydrogen fuel to flow adjacent to the anode catalyst ( 14 ), and a cathode flow path ( 38 ) is in fluid communication with a cathode catalyst ( 16 ) for directing an oxidant to flow adjacent to the cathode catalyst ( 16 ) of a fuel cell ( 12 ). Hydrogen fuel is permitted to transfer between the anode flow path ( 24 ) and the cathode flow path ( 38 ). A hydrogen reservoir ( 66 ) is secured in fluid communication with the anode flow path ( 24 ) for receiving and storing hydrogen during fuel cell ( 12 ) operation, and for releasing the hydrogen into fuel cell ( 12 ) whenever the fuel cell ( 12 ) is shut down.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 10/635,779, filed on Aug. 6, 2003.

TECHNICAL FIELD

The present invention relates to fuel cell power plants that are suitedfor usage in transportation vehicles, portable power plants, or asstationary power plants, and the invention especially relates to asystem that minimizes performance degradation of fuel cells of the plantresulting from repeated shutting down and starting up of the plant.

BACKGROUND ART

Fuel cell power plants are well-known and are commonly used to produceelectrical energy from hydrogen containing reducing fluid fuel andoxygen containing oxidant reactant streams to power electrical apparatussuch as power plants and transportation vehicles. In fuel cell powerplants of the prior art, it is well known that, when an electricalcircuit connected to the fuel cells is disconnected or opened and thereis no longer a load across the cell, such as upon and during shut downof the cell, the presence of air on a cathode electrode along withhydrogen fuel remaining on an anode electrode, often cause unacceptableanode and cathode potentials, resulting in oxidation and corrosion ofelectrode catalyst and catalyst support materials and attendant cellperformance degradation.

Passivation efforts have been proposed to return the cathode electrodeto a passive, non-oxidative state upon shut down of the fuel cell. Forexample, it was thought that inert gas needed to be used to purge bothan anode flow field and a cathode flow field immediately upon cell shutdown to passivate the anode and cathode electrodes so as to minimize orprevent such cell performance degradation. Further, the use of an inertgas purge avoided, on start-up, the possibility of the presence of aflammable mixture of hydrogen and air, which is a safety issue. Commonlyowned U.S. Pat. Nos. 5,013,617 and 5,045,414 describe using 100%nitrogen as the anode side purge gas, and a cathode side purging mixturecomprising a very small percentage of oxygen (e.g. less than 1%) with abalance of nitrogen. Both of these patents also discuss the option ofconnecting a dummy electrical load across the cell during the start of apurging process to lower the cathode potential rapidly to between theacceptable limits of 0.3-0.7 volt. However, the costs and complexity ofsuch stored inert gases are undesirable especially in automotiveapplications where compactness and low cost are critical, and where thesystem must be shut down and started up frequently.

Other efforts to minimize corrosion of catalyst and catalyst supportmaterials include shutting down a fuel cell power plant by disconnectingthe primary electricity using device (hereinafter, “primary load”),shutting off the air or process oxidant flow, and controlling thehydrogen fuel flow into the system and the gas flow out of the system ina manner that results in the fuel cell gases coming to equilibriumacross the cells, and maintaining a gas composition of at least 0.0001%hydrogen (by volume), balance fuel cell inert gas, during shut down.This method of fuel cell shut down also includes, after disconnectingthe primary load and shutting off the air supply to the cathode flowfield, continuing to supply fresh fuel to the anode flow field until theremaining oxidant is completely consumed. This oxidant consumption ispreferably aided by having a small auxiliary load applied across thecell, which also quickly drives down the electrode potentials. Once allthe oxidant is consumed the hydrogen fuel feed is stopped. Thereafter,during continued shut down, a hydrogen concentration is monitored; andhydrogen is added, as and if necessary, to maintain the desired hydrogenconcentration level.

Known improvements to the problem of oxidation and corrosion ofelectrode catalysts and catalyst support materials have reduced thedeleterious consequences of the presence of oxygen on the cathodeelectrode and a non-equilibrium of reactant fluids between the anode andcathode electrodes that result in unacceptable anode and cathodeelectrode potentials upon and during shut down and start up of a fuelcell. However, it has been found that even with known solutions, thepresence of oxygen within an anode flow field during start up results ina reverse current leading to unacceptable, localized electrodepotentials and corrosion of catalysts and catalyst support materials.Moreover, active addition of hydrogen to fuel cells of a power plantwhile the plant is shut down and unattended presents significant safetyissues where a system failure may lead to release of potentiallyflammable hydrogen concentrations out of the power plant.

Consequently, there is a need for a shut down system for a fuel cellpower plant that eliminates significant performance degradation of theplant, and that minimizes oxidation and corrosion within plant fuelcells at shut down of the plant, during shut down, or upon restartingthe fuel cell power plant.

DISCLOSURE OF INVENTION

The invention is a hydrogen passivation shut down system for a fuel cellpower plant. The system includes at least one fuel cell for generatingelectrical current from hydrogen containing reducing fluid fuel andprocess oxidant reactant streams. The fuel cell includes an anodecatalyst and a cathode catalyst on opposed sides of an electrolyte; ananode flow path in fluid communication with the anode catalyst fordirecting the hydrogen fuel to flow through the fuel cell and to flowadjacent to the anode catalyst; and a cathode flow path in fluidcommunication with the cathode catalyst for directing the oxidant toflow through the fuel cell and to flow adjacent to the cathode catalyst.A hydrogen inlet valve is secured between a hydrogen containing reducingfluid fuel storage source and the anode flow path for selectivelypermitting the hydrogen fuel to flow into the anode flow path. Anoxidant inlet valve is secured between an oxygen containing oxidantstorage source and the cathode flow path for selectively permitting theoxidant to flow into the cathode flow path.

The system includes hydrogen transfer means secured in communicationbetween the anode flow path and the cathode flow path for selectivelypermitting transfer of the hydrogen fuel between the anode flow path andthe cathode flow path. The hydrogen transfer means may be in the form ofa hydrogen transfer valve in fluid communication between the anode andcathode flow paths, an electrochemical pump for pumping hydrogen fromthe anode flow path through the electrolyte into the cathode flow path,or a proton exchange membrane (“PEM”) electrolyte that permits diffusionof hydrogen from the anode flow path through the PEM electrolyte intothe cathode flow path. Additionally, a hydrogen reservoir is secured influid communication with the anode flow path. The hydrogen reservoirreceives and stores hydrogen whenever the hydrogen inlet valve is opento permit flow of the hydrogen fuel through the anode flow path, and thehydrogen reservoir releases the hydrogen into the anode flow pathwhenever the hydrogen inlet valve is closed and the hydrogenconcentration in the anode flow field is reduced below a hydrogenconcentration during operation of the fuel cell. The hydrogen reservoirmay be hydrogen storage media, such as hydrides, that are located withinthe anode flow path, such as coatings on manifolds within the anode flowpath, or located within porous support plates supporting or in fluidcommunication with the anode catalyst. The hydrogen reservoir may alsobe a hydrogen vessel secured outside of the fuel cell that may also havehydrogen storage media within the vessel.

In use of a preferred embodiment of the system, whenever the fuel cellis shut down, the oxidant inlet valve is closed to prohibit the oxidantfrom flowing into the cathode flow path, the oxygen within the cathodeflow path is consumed, and then the hydrogen transfer valve is opened topermit hydrogen fuel from the fuel storage source and stored hydrogenwithin the hydrogen reservoir to move into the cathode flow path. Whenthe cathode and anode flow paths are substantially filled with about100% hydrogen, the hydrogen inlet valve is closed, and any hydrogenexhaust and oxidant exhaust valves are closed. During a shut downperiod, some oxygen from the atmosphere will enter the fuel cell, andhydrogen stored within the hydrogen reservoir continues to move from thereservoir into the anode and cathode flow paths to react with the oxygenand maintain a finite concentration in excess of 0.0001 percent hydrogenwithin the flow paths.

In another preferred embodiment of the system, a cathode recycle lineincluding a cathode recycle blower and an oxidant blower may be securedin fluid communication between a cathode exhaust and cathode inlet ofthe cathode flow path. During a shut down procedure, the cathode recycleor oxidant blower may be operated after an oxidant source isolationvalve is closed to rapidly cycle the hydrogen fuel from the anode flowpath, through the hydrogen transfer valve and into and throughout thecathode flow path.

In an additional embodiment, the system may include a hydrogen sensorthat may be utilized to determine a concentration of hydrogen fuelwithin the anode and cathode flow paths while the fuel cell power plantis shut down. If the sensor detects that the hydrogen concentration hasdeclined below acceptable limits, such as below 0.0001 percent hydrogen,a controller may open the hydrogen inlet valve to actively directhydrogen to enter the anode and cathode flow paths, while the fuel cellpower plant is shut down, such as immediately prior to a start up of theplant. Output from the sensor may also be used to select a start upprocedure. An exemplary start up procedure includes a rapid fuel purgewherein the hydrogen fuel is directed to traverse an anode flow field ofthe fuel cell in less than 1.0 seconds, or preferably in less than 0.2seconds, and most preferably in less than 0.05 seconds to minimizeoxidation and corrosion of electrode catalyst and catalyst supportmaterials. The hydrogen sensor may be a direct hydrogen concentrationsensor known in the art, or a sensor circuit in electrical communicationwith the catalysts of the fuel cell.

The system may also include an anode recycle line and anode recycleblower secured in fluid communication between an anode exhaust and anodeinlet of the anode flow path. The anode recycle line and blower may alsobe in fluid communication with the reducing fluid fuel storage source sothat the anode recycle blower may rapidly move the hydrogen fuel throughthe anode flow path.

In a further embodiment, the anode flow path may include an anodeexhaust vent, and the cathode flow path may include a cathode exhaustvent, wherein both the anode exhaust vent and cathode exhaust vent arelocated with reference to a directional force of gravity to be below thefuel cell. Because hydrogen is lighter than oxygen, the hydrogen willtend to remain above, or within the fuel cell while atmospheric oxygenentering the flow paths during shut down will tend to flow downward, outof the anode and cathode flow paths through the anode and cathodeexhaust vents, thereby aiding in preserving a finite hydrogenconcentration of greater than 0.0001 percent during shut down of thefuel cell power plant.

Accordingly, it is a general purpose of the present invention to providea hydrogen passivation shut down system for a fuel cell power plant thatovercomes deficiencies of the prior art.

It is a more specific purpose to provide a hydrogen passivation shutdown system for a fuel cell power plant that substantially fills andmaintains an anode flow path and cathode flow path of the plant withabout 100 percent hydrogen during shut down of the plant to therebypassivate fuel cell cathode and anode catalysts and catalyst supportmaterials while the fuel cell power plant is shut down.

It is yet another purpose to provide a hydrogen passivation shut downsystem for a fuel cell power plant that senses hydrogen concentrationswithin an anode flow path and a cathode flow path of the plant duringshut down of the plant and that permits additional hydrogen to enter theflow paths prior to start up of the plant to passivate fuel cell cathodeand anode catalysts and catalyst support materials

These and other purposes and advantages of the present hydrogenpassivation shut down system for a fuel cell power plant will becomemore readily apparent when the following description is read inconjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a preferred embodiment of ahydrogen passivation shut down system for a fuel cell power plantconstructed in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings in detail, a first embodiment of a hydrogenpassivation shut down system for a fuel cell power plant is shown inFIG. 1, and is generally designated by the reference numeral 10. Thesystem 10 includes at least one fuel cell, such as a fuel cell 12 havingan anode catalyst 14 (which may also be referred to herein as an anodeelectrode), a cathode catalyst 16 (which may also be referred to as acathode electrode), and an electrolyte 18 disposed between the anode andcathode. The electrolyte 18 may be in the form of a proton exchangemembrane (PEM) of the type described in U.S. Pat. No. 6,024,848, or theelectrolyte may be held within a ceramic matrix, such as is typicallyfound in acid aqueous electrolyte fuel cells, such as phosphoric acidelectrolyte fuel cells.

The anode catalyst 14 may be supported on an anode substrate layer 20,and the cathode electrode 16 may be supported on a cathode substratelayer 22. The system 10 also includes an anode flow path 24 in fluidcommunication with the anode catalyst 14 for directing a hydrogencontaining reducing fluid fuel to pass from a fuel source 54 through thefuel cell 12 and adjacent to the anode catalyst 14. The anode flow path24 includes an anode inlet 26 for directing the hydrogen fuel into thefuel cell 12, such as manifolds etc. known in the art. The anode inlet26 is in fluid communication with an anode flow field 28, which is partof the anode flow path 24, and is defined as voids, channels, or poresof support material, in fluid communication with and adjacent to theanode catalyst 14 for directing the hydrogen fuel to pass adjacent tothe anode catalyst 14. The anode flow path 24 also includes an anodeexhaust 30, in fluid communication with the anode flow field 28, fordirecting the hydrogen fuel out of the fuel cell 12. An anode exhaustvalve 32 is secured in fluid communication with the anode exhaust 30,and an anode exhaust vent 34 is secured to the anode exhaust valve 32.An anode vacuum release valve 36 in the nature of a known one-way, orcheck valve may be secured to the anode exhaust 30, to an anode recycleline 75, or to the anode flow path 24 to permit atmospheric air to moveinto the anode flow path 24 to avoid a partial vacuum forming within theanode flow path 24 during shut down of the fuel cell 12 as gases areconsumed in reactions, or condensed, as is known in the art.

The system 10 also includes a cathode flow path 38 in fluidcommunication with the cathode catalyst 16 for directing an oxygencontaining oxidant to pass through the fuel cell 12 and adjacent to thecathode catalyst 16. The cathode flow path 38 includes a cathode inlet40 for directing the oxidant into the fuel cell 12, such as manifoldsetc. known in the art. The cathode inlet 40 is in fluid communicationwith a cathode flow field 42, which is part of the cathode flow path 24,and is defined as voids, channels, or pores of support material, influid communication with and adjacent to the cathode catalyst 16 fordirecting the oxidant to pass adjacent to the cathode catalyst 16. Thecathode flow path 38 also includes a cathode exhaust 44, in fluidcommunication with the cathode flow field 42, for directing the oxidantout of the fuel cell 12. A cathode exhaust valve 46 is secured in fluidcommunication with the cathode exhaust 44, and a cathode exhaust vent 48is secured to the cathode exhaust valve 44. A cathode vacuum releasevalve 50 in the nature of a known one-way, or check valve may be securedto the cathode exhaust 44, or to the cathode flow path 38 to permitatmospheric air to move into the cathode flow path 38 to avoid a partialvacuum forming within the cathode flow path 38 during shut down of thefuel cell 12 as gases are consumed in reactions, or condensed, as isknown in the art.

It is pointed out that the anode exhaust vent 34 and cathode exhaustvent 48 are both disposed below the fuel cell 12, wherein “below” isassociated with a reference to a directional force of gravity asrepresented by a directional arrow 53 shown in FIG. 1. By having theanode exhaust vent 34 and cathode exhaust vent 48 disposed to dischargegases from the anode flow path 24 and cathode flow path 38 below thefuel cell 12, hydrogen gas being lighter than oxygen will tend to riseabove the oxygen and remain within the fuel cell 12 while heavier oxygenwill tend to flow in the direction of gravity 53 through the vents 34,48 prior to any hydrogen passing through the vents 34, 48. The anodeexhaust vent 34 and the cathode exhaust vent 48 may also be in the formof vacuum release valves that prevent a vacuum from forming inside thefuel cell 12.

A hydrogen inlet valve 52 is secured in fluid communication between theanode inlet 26 of the anode flow path 24 and the hydrogen containingreducing fluid fuel storage source 54 for selectively directing thehydrogen fuel to flow into the anode flow path 24. A hydrogen fuel feedline 55 may be secured between the hydrogen fuel source 54 and thehydrogen inlet valve 52. An oxidant inlet valve 56 is secured in fluidcommunication between an oxygen containing oxidant source 58, such asthe atmosphere, and the cathode inlet 40 for selectively directing theoxidant to flow into the cathode flow path 38. An oxidant blower orcompressor 60 may be secured to an oxidant feed line 62 between theoxidant source 58 and the oxidant inlet valve 56 for pressurizing theoxidant as it moves into and through the cathode flow path 38. Theoxidant inlet valve 56 may be located upstream of the blower 60, ordownstream of the oxidant blower 60 (as shown in FIG. 1).

The system also includes hydrogen transfer means in communicationbetween the anode flow path 24 and the cathode flow path 38 forselectively permitting transfer of hydrogen fuel between the anode flowpath 24 and the cathode flow path 38 during shut down of the fuel cell12. The hydrogen transfer means may be a hydrogen transfer valve 64secured in fluid communication between the anode flow path 24 and thecathode flow path 38, such as between the anode inlet 26 and the cathodeinlet 40. By use of the phrase “for selectively” permitting ordirecting, it is meant herein that a switch or valve, such as thehydrogen transfer valve 64 may be selected to be in an open position tothereby permit flow of the hydrogen fuel between the anode flow path 24and the cathode flow path 38, or the valve 64 may be selected to be in aclosed position to prohibit flow of the hydrogen fuel or any fluidbetween the anode and cathode flow paths 24, 38.

Alternatively, the hydrogen transfer means may also be in the form of anelectrochemical hydrogen pump, wherein hydrogen is electrochemicallypumped from the anode flow path 24 to the cathode flow path 38 bypassing a direct current through the fuel cell in a manner known in theart so that hydrogen is consumed at the anode catalyst 14 and evolved atthe cathode catalyst 16 to increase a concentration of hydrogen in thecathode flow field 42. Such a hydrogen transfer electrochemical pumpreduces an oxygen concentration within the cathode flow path 38 duringshut down of the fuel cell 12 and reduces a requirement for additionalvalves and plumbing to achieve the reduced oxygen concentration. Thehydrogen transfer means may also be in the form of a hydrogen transferproton exchange membrane (“PEM”) electrolyte 18, wherein hydrogendiffuses across the PEM electrolyte 18 until the hydrogen concentrationwithin the cathode flow field 42 is in substantial equilibrium with thehydrogen concentration with the hydrogen concentration within the anodeflow field 28. Such a hydrogen transfer means transfers hydrogen at aslower rate than the previously described hydrogen transfer valve 64 andhydrogen transfer electrochemical pump, but the hydrogen transfer PEMelectrolyte is the least complicated hydrogen transfer means.

The system 10 also includes hydrogen reservoir means for storing thehydrogen fuel secured in fluid communication with the anode flow path24. The hydrogen reservoir means may be in the form of a hydrogen vessel66 secured outside of the fuel cell 12 (as shown in FIG. 1.) to be influid communication with the anode flow path 24, such as through avessel feed line 68 being secured between the vessel 66 and the anodeinlet 26 of the anode flow path 24.

Alternatively, the hydrogen reservoir means may be in the form ofhydrogen storage media, such as hydrides that are secured within theanode flow path 24, such as by a coating. Additionally, the hydrogenstorage media may be applied as a coating of pores of the porous anodesubstrate layer 20, so that hydrogen fuel is stored within the storagemedia as the fuel flows through the anode flow path 24. Also, thehydrogen vessel 66 may include hydrogen storage media within the vessel66. The hydrogen storage media may also be in the form of a coating ofinlet or exhaust manifolds defined within the anode inlet 26 or anodeexhaust 30 so that the hydrogen storage media is in fluid communicationwith the hydrogen fuel passing through the anode flow path 24. Thehydrogen storage media of the hydrogen reservoir means may also be acoating within the anode flow field 28 exposed to the hydrogen fuel. Thehydrogen reservoir means for storing hydrogen fuel thus is able to storethe hydrogen fuel as the fuel passes through the anode flow path 24 andthe media may passively release the stored hydrogen into the anode flowpath 24 whenever the hydrogen fuel is no longer passing from thehydrogen fuel storage source 52 through the anode flow path 24. Thehydrogen reservoir means and hydrogen transfer means may be constructedso that the system 10 may achieve a hydrogen concentration in the anodeflow path 24 and cathode flow path 38 of substantially pure hydrogen,wherein “substantially pure hydrogen” is a hydrogen concentration ofgreater than seventy percent hydrogen, or alternatively the system mayachieve a concentration within the anode flow path 24 and cathode flowpath 24 of essentially pure hydrogen, wherein “essentially purehydrogen” is a hydrogen concentration of greater than ninety percenthydrogen.

The hydrogen passivation shut down system for a fuel cell power plant 10also may include a first cathode recycle line 70 secured in fluidcommunication between the cathode exhaust 44 of the cathode flow path 38and the oxidant feed line 62 upstream of the blower 60 and downstream ofan oxidant source isolation valve 71 as shown in FIG. 1. A cathoderecycle valve 72 may selectively permit a portion of a cathode exhauststream to pass from the cathode exhaust 44 to the oxidant feed line 62to pass again through the cathode flow path 38. When the oxidant sourceisolation valve 71 is closed, the cathode recycle blower 76 or theoxidant blower 60 may be operated continuously or intermittently duringthe shutdown process to accelerate a rate of oxygen reduction from thecathode flow path 38, which includes the cathode flow field 42 andassociated inlet and exit manifolds and plumbing known in the art. Inthe absence of such a recycle flow the oxygen contained within thecathode flow path 38 manifolds would slowly diffuse into the cathodeflow field 42 where it would react with hydrogen on the cathode catalyst16. That reaction with the hydrogen would consume the hydrogen, therebyreducing the time the fuel cell 12 could be maintained in a passivestate. Recycling hydrogen from the hydrogen reservoir means 66 throughthe first cathode recycle line 70 and cathode flow path 38 maximizes ahydrogen concentration of the fuel cell 12 at the end of a fuel cell 12shut down process. That in turn maximizes a duration the fuel cell 12can be maintained in a passive state without adding additional hydrogento the fuel cell 12.

A second cathode recycle line 74 may be secured in fluid communicationbetween the cathode recycle valve 72 and the cathode inlet 40, and acathode recycle blower 76 may be secured to the second cathode recycleline 74 to accelerate flow through the second cathode recycle line. Thesystem 10 may also include an anode recycle line 75 secured in fluidcommunication between the anode exhaust 30 and the anode inlet 26,having an anode recycle blower 77 secured to the anode recycle line 75to accelerate flow through the anode recycle line 75.

The system 10 may also include hydrogen sensor means for detecting aconcentration of hydrogen within the anode flow path 24 and the cathodeflow path 38. The hydrogen sensor means may be a direct hydrogen sensor78 or sensors known in the art secured, for example, in the cathode flowfield 42 for sensing and communicating to a controller the hydrogenconcentration within the cathode flow path 38 when the fuel cell powerplant 10 is shut down. Such a controller may be any controller means(not shown) known in the art capable of receiving and responding tosensed information, such as a computer, electromechanical switches, ahuman controller, etc.

Alternatively, the hydrogen sensor means may be a sensor circuit 80secured in electrical communication with the cathode catalyst 14 andanode catalyst 16 of the fuel cell 12, such as through an externalcircuit 82. The sensor circuit 80 includes a direct current power source84 such as a D.C. conventional, regulated power supply, battery-type ofpower source; a voltage-measuring device means for measuring the voltagein the sensor circuit, such as a standard voltmeter 86; and a sensorcircuit switch 88. The sensor circuit 80 is calibrated by establishingthe voltage, at a fixed current, as a gas composition in both the anodeflow field 28 and cathode flow field 42 is varied from pure hydrogen toair. The sensor circuit 80 may selectively deliver a pre-determinedsensing current to the fuel cell 12 for a pre-determined sensingduration for measuring a voltage difference between the anode catalyst14 and cathode catalyst 16 to thereby determine hydrogen concentrationswithin the anode flow path 24 and cathode flow path 38.

During normal operation of the fuel cell power plant 10, a primary load90 receives electrical current generated by the fuel cell 12 through theexternal circuit 82, and a primary load switch 92 is closed (it is shownopen in FIG. 1); an auxiliary load 94 does not receive electricalcurrent and an auxiliary load switch 96 is open, so that the fuel cellpower plant 10 is providing electricity only to the primary load 90,such as an electric motor, etc.; and the sensor circuit switch 88 isopen, so that the sensor circuit 84 is not directing any electricalcurrent to the anode and cathode catalysts 14, 16. The oxidant blower60, and the anode exhaust recycle blower 77 are on. The oxidant inletvalve 56 and cathode exhaust valve 46 are open, as are the hydrogeninlet valve 52 and anode exhaust valve 32. The anode vacuum releasevalve 36 is closed so that no air flows into the anode flow path 24.

Therefore, during normal operation of the plant 10, process oxidant suchas air from the oxidant source 58 is continuously delivered into thecathode flow field 42 through the cathode flow path 38, and leaves thecathode flow path 38 through the cathode exhaust vent 48. The hydrogencontaining reducing fluid fuel from the fuel source 54 is continuouslydelivered into the anode flow field 28 through the anode flow path 24. Aportion of an anode exhaust stream, containing depleted hydrogen fuel,leaves the anode flow path 24 through the anode exhaust valve 32 and theanode exhaust vent 34, while the anode recycle line 75 and anode recycleblower 77 re-circulates the balance of the anode exhaust through theanode flow path 24 in a manner well know in the prior art. Recycling aportion of the anode exhaust helps maintain a relatively uniform gascomposition throughout the anode flow path 24, and permits increasedhydrogen utilization. As the hydrogen passes through the anode flowfield, it electrochemically reacts on the anode catalyst layer 14 in awell-known manner to produce protons (hydrogen ions) and electrons. Theelectrons flow from the anode catalyst 14 to the cathode catalyst 16through the external circuit 82 to power the primary load 90.

Shutting down the operating fuel cell power plant 10 includes opening ordisconnecting the primary load switch 92 (as shown in FIG. 1) in theexternal circuit 82 to disconnect the primary load 90. The hydrogeninlet valve 52 remains open; and the anode exhaust recycle blower 77remains on to continue recirculation of a portion of the anode exhaust.However, the anode exhaust valve 32 will remain open or be closeddepending upon the percent hydrogen in the incoming fuel. The flow offresh air or oxidant through the cathode flow path 38 is turned off byturning off the cathode blower 60.

During shut down the auxiliary load 94 may then be connected to theexternal circuit 82 by closing the auxiliary load switch 96. Withcurrent flowing through the auxiliary load 94, typical electrochemicalcell reactions occur, causing the oxygen concentration in the cathodeflow path 38 to be reduced and cell voltage to be lowered. Theapplication of the auxiliary load 94 is initiated while there is stillsufficient hydrogen within the fuel cell 12 to electrochemically reactall the oxygen remaining within the fuel cell 12. It preferably remainsconnected at least until the cell voltage is lowered to a pre-selectedvalue, preferably 0.2 volts per cell or less. A diode 98, connectedacross the cathode catalyst 14 and anode catalyst 16, senses the cellvoltage and allows current to pass through the auxiliary load 94 as longas the cell voltage is above the pre-selected value. In that way, thefuel cell 12 voltage is reduced to and thereafter limited to thepre-selected value. When the cell voltage drops to 0.2 volts per cell,substantially all the oxygen within the cathode flow field 42, and anythat has diffused across the electrolyte 18 to the anode flow field 28,will have been consumed. The auxiliary load 94 may then be disconnectedby opening the auxiliary load switch 96, but is preferably leftconnected.

The hydrogen transfer valve 64 may then be selected to an open positionto permit hydrogen fuel to pass from the anode flow path 24 into thecathode flow path 38. The oxidant source isolation valve 71 is thenclosed, and the cathode recycle valve 72 may then be opened while thecathode recycle blower 76 or oxidant blower 60 is turned on to draw thehydrogen from the anode flow path 24 through the hydrogen transfer valve64 and through the cathode flow path 38. Whenever the hydrogen sensormeans determines that the concentration of hydrogen within the anodeflow path 24 and cathode flow path 38 is about one-hundred percent(100%) hydrogen, the anode exhaust valve 32 and cathode exhaust valve 46are closed, the hydrogen inlet valve 52, oxidant inlet valve 56, andcathode recycle valve 72 are also closed, while the hydrogen transfervalve 64 remains open. Hydrogen stored within the hydrogen reservoirmeans may then be passively released to maintain an elevated hydrogenconcentration within the anode flow path 24 and cathode flow path 38during shut down of the fuel cell power plant 10. It is desired tomaximize the hydrogen concentration within the anode flow path 24 andcathode flow path 38 during the shut down process. Maximizing thehydrogen concentration at the end of the shut down process will maximizea time the fuel cell 12 will be maintained in a passive state withoutthe addition of more hydrogen. A preferred hydrogen concentration atshut down is greater than seventy percent (70%) hydrogen, and a morepreferred hydrogen concentration is greater than ninety per cent (90%).During the shutdown period, it is preferred that the auxiliary load 94is connected to the external circuit 82 by closing the auxiliary loadswitch 96. This minimizes the potential of the individual electrode orcathode catalyst 16 and cathode substrate 22 should air leak into thefuel cell 12.

During shut down of the plant 10, oxygen from the air may leak into thecathode flow path 24 or anode flow path 38 through seals, or through theanode vacuum release valve 36 or cathode vacuum release valve 50 so thatthe potential of the anode and cathode catalysts 14, 16 will eventuallyascend above 0.2 volts relative to a hydrogen reference electrode,leading to oxidative decay within the fuel cell 12. Hydrogen gas fromthe reducing fluid source 54 may then be admitted prior to the electrodepotential reaching 0.2 volts in order to consume the oxygen, therebyminimizing any oxidative decay. The hydrogen may be circulatedthroughout the anode flow path 24 by opening the anode inlet valve 52and turning on the anode recycle blower 68 while the anode exhaust valve32 remains closed. Alternatively, an anode recycle valve 100 secured toan anode recycle feed line 102 secured in fluid communication betweenthe hydrogen fuel storage source 54 and the anode recycle line 75 may beopened to supply hydrogen to the anode flow path 24 while the hydrogeninlet valve 52 remains closed. Any such admitted hydrogen will also passthrough the hydrogen transfer means to pass into the cathode flow path38. The cathode recycle blower 76 or oxidant blower 60 may also be usedto hasten distribution of the hydrogen throughout the cathode flow path38. A quantity of hydrogen that is admitted to the flow paths 24, 38 maybe inversely proportional to a concentration of hydrogen within theanode and cathode flow paths 24, 38. That minimizes the quantity ofhydrogen that is required to maintain the fuel cell 12 in a passivestate and maximizes the time the fuel cell 12 can be maintained in apassive state without addition of hydrogen to the flow paths 24, 38during shut down of the plant 10.

The sensor circuit 80 may also be in communication with a hydrogenadmitting controller means (not shown) for controlling admission of thehydrogen fuel into the anode flow path 24 and cathode flow path 38. Thehydrogen admitting controller means may be any controller known in theart that can accomplish the task of admitting hydrogen into the flowpaths 24, 38 upon detection by the sensor circuit 80 of a shut downmonitoring voltage at about or exceeding the sensor voltage limit.Exemplary controller means include simple manual opening by a powerplant operator (not shown) of the hydrogen inlet valve 52, anode recyclevalve 100, or any other mechanism capable of admitting hydrogen into theflow paths 24, 38 and starting of the anode exhaust recycle blower 77 bythe operator or a control system. Other controller means could includeelectromechanical controls integrating the voltage measuring device withthe hydrogen inlet valve 52, anode recycle valve 100, as well as withthe anode recycle blower 68, cathode recycle blower, such as are knownin the art for opening valves and blowers, etc., in response to sensedsignals.

For example, in a passive method of using the system 10, an operator(not shown) may utilize the sensor means, such as the direct sensor 78,to determine if an adequate volume of hydrogen is within the anode andcathode flow paths 24, 39 immediately prior to starting up the fuel cell12 after a period of being shut down, such as an automobile powered bythe fuel cell being shut down over night. If the sensor 78 indicatesadequate hydrogen is present to maintain the anode electrode 14 andcathode electrode 16 potentials at an adequately low potential, such asless than 0.2 volts relative to a standard hydrogen electrode, than anordinary start up may be utilized, wherein the hydrogen transfer valve64 is closed, the hydrogen inlet valve 52, oxidant inlet valve 56 andisolation valve 71 are opened, the oxidant blower 60, is activated, theanode recycle blower 66 is activated, and the anode and cathode exhaustvalve 32, 46 are opened.

However, if the sensor means detects an inadequate concentration ofhydrogen, a rapid hydrogen purge may be utilized to eliminate oxygen incontact with the anode and cathode catalysts 14, 16 and the anode andcathode support substrate layers 20, 22. A rapid hydrogen fuel purgeincludes directing the hydrogen fuel to traverse the anode flow field 28from the anode inlet 26 to the anode exhaust 30 in less than 1.0seconds, or preferably in less than 0.2 seconds and most preferably inless than 0.05 seconds. Preferably the auxiliary load 94 is connectedduring the hydrogen purge. Air flow to the cathode flow field 42 isbegun after the hydrogen purge is completed and the auxiliary load 96removed. Such a rapid hydrogen fuel purge may be accomplished byutilization of a highly pressurized hydrogen fuel source 54 known in theart, or fuel blowers or compressors, etc. also known in the art. In thispassive usage of the system 10, hydrogen is only admitted to the fuelcell 12 while an operator is present, thereby eliminating safetyconcerns of unattended hydrogen transfer, wherein a system malfunctionmight lead to release of flammable concentrations of hydrogen from thepower plant 10.

In an alternative, active usage of the present hydrogen passivation shutdown system 10, the sensor means may be utilized to detect when thecathode and anode electrode 14, 16 potentials ascend above theacceptable level, and then the hydrogen admitting controller responds tothe sensed information from the sensor means to control the hydrogeninlet valve 52, or the anode recycle valve 100 to admit an adequateamount of hydrogen into the anode flow path 24 to reduce the electrodepotential back to or below an acceptable level.

For specific embodiments of the system 10, wherein operationalrequirements do not anticipate long term shut downs, or forcircumstances wherein the fuel cell 12 is adequately sealed to restrictunacceptable depletion of hydrogen, the system 10 may rely only upon thepassive release of stored hydrogen from the hydrogen reservoir means,such as the hydrogen vessel 66 as described above. In such anembodiment, the system 10 includes hydrogen passivation of the fuel cell12 through the steps of disconnecting the primary load 90 from the fuelcell; terminating admission of the oxidant into the cathode flow path 38from the oxidant source, such as by shutting off the oxidant blower;operating the hydrogen transfer means to permit passage of hydrogen fromthe anode flow path 24 into the cathode flow path 38; shutting off flowof the hydrogen fuel into the anode flow path 24 whenever the anode flowpath 24 and cathode flow path 38 are filled with a predetermined,adequate volume of hydrogen; and permitting release into the anode flowpath 24, hydrogen transfer valve and cathode flow path 38 of hydrogenstored within the hydrogen reservoir means, such as from the hydrogenvessel 66. Optionally, that embodiment of the system may also includeoperating the cathode recycle blower to more rapidly consume oxygenwithin the cathode flow path 38; and closing the anode and cathodeexhaust valves 32, 46 when the anode and cathode flow paths 24, 38 arefilled with hydrogen.

It can be seen that the present hydrogen passivation shut down systemfor a fuel cell power plant 10 provides for efficient, passivation ofthe fuel cell catalysts or electrodes 14, 16 that reduces oxidativecorrosion of the catalysts and catalyst support materials by replacingoxygen in the fuel cell 12 with hydrogen while shutting down the fuelcell 12, and for replenishing hydrogen prior to start up of the fuelcell, either passively or actively, depending upon requirements of thesystem 10.

While the present invention has been disclosed with respect to thedescribed and illustrated embodiments, it is to be understood that theinvention is not to be limited to those embodiments. Accordingly,reference should be made primarily to the following claims rather thanthe foregoing description to determine the scope of the invention.

1. A hydrogen passivation shut down system for a fuel cell power plant(10), the system comprising: a. at least one fuel cell (12) forgenerating electrical current from hydrogen containing reducing fluidfuel and oxygen containing oxidant reactant streams, the fuel cell (12)including an anode catalyst (14) and a cathode catalyst (16) on opposedsides of an electrolyte (18), an anode flow path (24) in fluidcommunication with the anode catalyst (14) for directing the hydrogenfuel to flow through the fuel cell (12) and adjacent the anode catalyst(14), and a cathode flow path (38) in fluid communication with thecathode catalyst (16) for directing the oxidant stream to flow throughthe fuel cell (12) and adjacent the cathode catalyst (14); b. a hydrogeninlet valve (52) secured between a hydrogen containing reducing fluidfuel source (54) and the anode flow path (24) for selectively permittingthe hydrogen fuel to flow into the anode flow path (24); c. an oxidantinlet valve (56) secured between an oxygen containing oxidant source(58) and the cathode flow path (38) for selectively permitting theoxidant to flow into the cathode flow path (38); d. hydrogen transfermeans secured in communication between the anode flow path (24) and theoxidant flow path (38) for selectively permitting flow of the hydrogenfuel between the anode flow path (24) and the cathode flow path (38);and, e. hydrogen reservoir means secured in fluid communication with theanode flow path (24) for storing the hydrogen fuel whenever the hydrogeninlet valve (52) is open to permit flow of the hydrogen fuel through theanode flow path (24), and for releasing hydrogen fuel into the anodeflow path (24) whenever the hydrogen inlet valve (52) is closed.
 2. Thesystem of claim 1, wherein the hydrogen reservoir means comprises ahydrogen vessel (66) secured outside the fuel cell (12) in fluidcommunication with the anode flow path (24), the hydrogen vesselincluding a hydrogen storage media stored within the vessel (66).
 3. Thesystem of claim 1, wherein the hydrogen reservoir means comprises ahydrogen storage media secured in fluid communication with the anodeflow path (24).
 4. The system of claim 1, wherein the hydrogen reservoirmeans comprises a hydrogen storage media secured within the anode flowpath (24).
 5. The system of claim 1, further comprising a hydrogensensor means secured in communication with the fuel cell (12) fordetecting a concentration of hydrogen within the anode flow path (24)and the cathode flow path (38).